Such radiation detectors are known for example from French patent FR 2 605 166 in which a photosensitive sensor formed from amorphous silicon photodiodes is combined with a radiation converter.
The operation and the structure of such a radiation detection will be briefly recalled.
The photosensitive sensor is generally made from solid-state photosensitive elements arranged in a matrix. The photosensitive elements are made from semiconductor materials, usually single-crystal silicon for sensors of the CCD or CMOS type, polycrystalline silicon or amorphous silicon. A photosensitive element comprises at least one photodiode, phototransistor or photoresistor. These elements are deposited on a substrate, generally a glass plate.
In general, these elements are not directly sensitive to very short wavelength radiation, such as X-rays or gamma rays. This is why the photosensitive sensor is combined with a radiation converter, which includes a layer of a scintillating substance. This substance has the property, when it is excited by such radiation, of emitting radiation of longer wavelength, for example visible light or light near the visible, to which the sensor is sensitive. The light emitted by the radiation converter illuminates the photosensitive elements of the sensor, which perform a photoelectric conversion and deliver electrical signals that can be exploited by suitable circuits. In the rest of the description the radiation converter will be called a scintillator.
Certain scintillating substances of the family of alkali metal halides or rare-earth oxysulfides are frequently employed for their good performance characteristics.
Among alkali metal halides, cesium iodide, doped with sodium or thallium depending on whether emission at around 400 nanometers or at around 550 nanometers is desired respectively, is known for its strong X-ray absorption and for its excellent fluorescence yield. It is in the form of fine needles that are grown on a support. These needles are approximately perpendicular to this support and partly confine the light emitted toward the sensor. Their fineness determines the resolution of the detector. Lanthanum and gadolinium oxysulfides are also very widely employed for the same reasons.
However, among these scintillating substances, some have the drawback of being not very stable—they partly decompose when exposed to moisture and their decomposition releases chemical species that migrate either toward the sensor or away from the sensor. These species are highly corrosive. Cesium iodide and lanthanum oxysulfide in particular have this drawback.
As regards cesium iodide, its decomposition gives cesium hydroxide Cs+OH− and free iodine I2, which can then combine with iodide ions to give the complex I3−.
As regards lanthanum oxysulfide, its decomposition gives hydrogen sulfide H2S, which is chemically very corrosive.
The degradation of the scintillating substance may especially be responsible for the appearance of leakage currents in the photodetection matrix structure, which leakage currents may cause visible and in addition evolving impairment of the image produced by the detector.
Moisture is extremely difficult to eliminate. The ambient air, and the cement used for assembling the detector, always contain moisture. The presence of moisture in the cement is due either to the ambient air or to the polymerization reaction, if this results from the condensation of two chemical species, which is frequently the case.
One of the important aspects when producing these detectors is to minimize the amount of moisture, initially present inside the detector and in contact with the scintillator, and to prevent this moisture from diffusing into the sensor during its operation.
The radiation detectors have an entry window through which the X-rays upstream of the scintillator pass. Moreover, the schintillating substance is generally deposited on a metal substrate. The substrate and the schintillating substance then form the scintillator. In addition, it is known to use the substrate as entry window.
When the schintillating substance is deposited directly on the entry window in order to form the scintillator, which is then attached to the sensor, the entry window must withstand, without being damaged, the thermal stresses arising from the deposition and treatment of the scintillator and must preferably have an expansion coefficient of the same order of magnitude as that of the scintillator and that of the sensor, more particularly that of its substrate. Provision may also be made for the window to have a low elastic modulus, thereby making it possible to eliminate differential strains between, on the one hand, the window and the scintillator and, on the other hand, the window and the sensor, or more particularly the sensor substrate. This thus eliminates the risks of the scintillator cracking and the sensor substrate fracturing.
It has also been sought to separate the entry window and scintillator substrate functions by adding an additional entry window that provides only the function of sealing the detector. The stresses to which the scintillator substrate was subjected are then distributed between the substrate and the additional entry window. The scintillator substrate remains subject to the same reflectivity and surface-finish constraints for scintillator deposition as in the prior art. However, it is no longer subject to the sealing and support constraints of the seal. These constraints are transferred to the additional entry window. Such a construction is disclosed in patent application FR 01/13899 filed on Oct. 26, 2001 in the name of the Applicant.
These precautions for improving the sealing of the detector are not sufficient to completely stop the scintillating substances from decomposing. It has been realized that the migration of ionic species is promoted by the existence of an electric field induced between the matrix of photosensitive elements and the scintillator substrate during operation of the detector.